The OT cast filmed for too long to be mere cameos - even if most of that filming were the action scenes which can probably take a long time to film. Outside of action, I expect Stewart and McKellen at least to have decent dramatic parts because after all Magneto and Professor X are the only characters who link the two time periods as young and old characters (Wolverine doesn't count since we're not likely to get to know 70s Logan much, he's only there for his body ).

I just hope it won't be like The Hobbit where they throw original LoTR characters like Elrond, Galadriel and Saruman onscreen for nostalgia's sake who don't actually get to do much.